Pharmacokinetics and pharmacodynamics for VNs
Abstract
Pharmacodynamics is defined as the science of drug action on the body and pharmacokinetics is defined as the study of what the body does to a drug, namely the study of drug absorption, distribution, metabolism and elimination. A basic understanding of pharmacokinetics and pharmacodynamics is important, particularly in the discipline of anaesthesia. It allows known and unfamiliar drugs to be used more safely and efficiently and ensures a greater understanding of clinical effect. It allows predictions to be made about the clinical characteristics of anaesthetic and analgesic drugs that may be unfamiliar, increasing understanding about how the drug performs in both healthy and unhealthy patients. Pharmacokinetic properties will also inform about whether individual drugs are suitable for administration by continuous rate infusion.
A second article focussing on the principles of pharmacodynamics will appear in a future issue of The Veterinary Nurse.
Pharmacokinetics is the study of the processes of drug absorption, distribution and elimination, either by metabolism or excretion. Detailed pharmacokinetic studies quantify these events and their time course.
The majority of drugs do not exert their effects in the compartment into which they are delivered, e.g. the gastrointestinal tract for drugs given orally or the plasma for drugs given intravenously. Drugs need to cross lipid cell membranes in order to reach their site of action and this can be achieved in a number of ways:
(a weak acid will give up a hydrogen ion in water and become ionized)
(a weak base will accept a hydrogen ion in water and become ionized)
When in solution the ionized form of the drug is present in equilibration with the unionized form, how much of the drug changes to the ionized form depends on the pKa of the drug and the pH of the solution. The pKa of the drug is related to the equilibrium that the drug has with its ionized form and is the pH of the solution at which 50% of the drug will be ionized and 50% will be unionized. Therefore when the pH of the solution is the same as the pKa of the drug then 50% of the drug will be ionized and 50% of the drug will be unionized.
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